1. Field of Invention
This invention relates generally to optical information storage and retrieval systems and, more particularly, to the read/write heads of the optical information storage and retrieval systems. The read/write head generates and receives the optical beam which interacts with the storage medium.
2. Description of the Related Art
The use of diffraction grating couplers in the read/write heads of optical information storage and retrieval systems can provide distinct advantages over similar apparatus used in the prior art. The advantages of the diffraction gratings include such features as reduced weight and a compatibility with integrated optical systems. However, the diffraction grating couplers have a wavelength dependence which can compromise the coupling efficiency. In optical storage and retrieval systems, the radiation is typically furnished by a semiconductor laser diode. Semiconductor laser diodes exhibit shifts in operating wavelength which are a function of temperature and power level.
In the past, two approaches for minimizing the effects of changes in the semiconductor diode operating wavelength have been employed. In the first approach, an attempt Was been made to improve the wavelength stability of laser diodes without expensive (and bulky) temperature control circuits. As a result of this effort, distributed feedback lasers have been developed which exhibit wavelength shifts of less than 3 nanometers (nm) over normal operating temperature ranges (i.e., 20.degree. C. to 60.degree. C.). In the second approach, the optical system is designed to include a wavelength shift compensation mechanism. Typically, the compensation mechanism has involved matching the dispersion of another optical element, such as a surface relief grating or volume hologram, with the dispersion of an integrated optic grating coupler.
Recently, it has been shown that a surface relief transmission grating can theoretically be used to achromatize a grating coupler over a 10 nm wavelength range, while a hybrid prism/grating coupler (i.e., a grating fabricated on a prism surface) can compensate for wavelength shifts in the range of over 200 nm. However, the surface relief transmission grating has an unblazed efficiency of less than 50%. The use of the hybrid prism grating coupler has the disadvantage of the bulk and the difficulty in retaining contact to the waveguide in the optical head. The transmission grating achromization approach is most practical when the output grating coupler configuration is considered. For the input radiation beam coupling, the resulting compensation will be over a shorter wavelength. The shorter wavelength coupling range is the result of a shift of the diffracted beam on the grating coupler. The beam shift will effect the coupling efficiency.
The grating coupler should be designed with a reasonable and uniform coupling efficiency. For traditional waveguide couplers, the maximum coupling efficiency is 80% for an incident Gaussian beach. As a practical consideration, a 50% coupling efficiency is considered good. It is therefore desirable that, in view of the losses in the grating coupler, the compensating apparatus be as efficient as possible.
Because of their high efficiency, transmission volume holograms have been considered as compensating elements. However, over wide wavelength ranges (&gt;10 nm), the efficiency of the transmission volume holograms can have unacceptable variation. In addition, the high efficiency occurs near the Bragg angle. This feature eliminates the freedom to tilt a compensation grating, a degree of freedom that can be used in adjustments.